1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a transfer sheet) according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Market demand for high-speed image forming apparatuses requires that a toner image be fixed on a recording medium properly in the fixing device even when the image forming apparatus forms the toner image on the recording medium at high speed with the shortened warm-up time period and first print time period.
To address such demand, the fixing device may include a heating member such as a heat-conductive metal pipe provided inside a loop formed by an endless belt and facing an inner circumferential surface of the belt. A heater heats the heating member so that the heating member heats the whole belt. A pressing rotary member located outside the loop formed by the belt is pressed against a fixed member fixedly provided inside the loop formed by the belt via the belt to form a nip portion between the pressing rotary member and the belt. The heating member includes an opening opposing the nip portion so that the heating member faces the inner circumferential surface of the belt at a position other than the nip portion, and is heated by the heater provided inside the heating member. With such a structure, a recording medium bearing a toner image passing through the nip portion receives heat from the belt heated by the heating member and pressure from the pressing rotary member to fix the toner image on the recording medium.
A clearance is provided between the fixed member and the heating member. Accordingly, even when the heating member has a thin thickness to improve heating efficiency, the heating member is not deformed by pressure applied to the fixed member by the pressing rotary member via the belt at the nip portion. However, when a lubricant is applied between the heating member and the belt to decrease resistance generated between the heating member and the belt sliding over the heating member, the lubricant may get into the heating member through the opening in the heating member opposing the nip portion. Consequently, a shortage of the lubricant may accelerate wear of the heating member and the belt, and the lubricant entering the heating member may adhere to the heater, resulting in degradation of the heater.
To address these problems, the heating member may have an endless loop shape corresponding to the belt without the opening. Instead of the fixed member, a reinforcement member may be provided inside the heating member and pressed against the pressing rotary member via the heating member and the belt to reinforce the heating member at the nip portion.
However, only the belt is provided between the heating member and the pressing rotary member, and therefore pressure from the pressing rotary member applies a substantial impact to the heating member. Accordingly, when the heating member has a thinner thickness to improve heating efficiency or when the pressing rotary member applies a greater pressure to the heating member via the belt to enlarge the nip portion so as to improve fixing efficiency, the heating member may be deformed. Consequently, a part of the belt may contact the heating member tightly, damaging the belt or generating noise. Further, deformation of the heating member may generate variation in the pressure applied to the heating member or may impact the heating member whenever the pressing rotary member contacts to and separates from the belt, neither of which is desirable.